When substrates are not available or are very expensive in self-supporting bulk form, they may be obtained as thinner layers by epitaxial growth on seed substrates. However, the properties of these seed substrates are not always perfectly matched to the materials that are desired to grow. For example, the seed substrates may have a thermal expansion coefficient and a lattice parameter that are different from those of materials that are desired to be grown. This may result in the epitaxially grown layer, hereinafter called the “epilayer,” being strained, either in compression or in tension. Alternatively, a number of defects, such as dislocations or cracks, may develop during growth of the structure or as it is being cooled, thereby reducing the efficacy of the devices subsequently formed on these layers.
Techniques for relaxing such layers of strained material are known, especially by introducing a low-viscosity layer between the strained layer and a support substrate. By applying a heat treatment at or above the glass transition temperature of the low-viscosity layer, it is then possible to relax the strained layer. However, these techniques do not give completely satisfactory results—the strained layer does not always or completely relax in an elastic mode. The structure formed from a stack of layers having different thermal expansion properties may also deteriorate and the layer to be relaxed may be pulled off the rest of the structure. In addition, when the material is compressively strained, the elastic relaxation may result in the material wrinkling, the roughness and the amplitude between peaks and valleys of the wrinkled layer then not being reconcilable with the desired uses. When the material is tensilely strained, the relaxation often creates cracks and greater surface roughness.
U.S. Patent Publication No. 2007/0072324, now U.S. Pat. No. 8,334,155, issued Dec. 18, 2012, by Krames et al., describes the formation of trenches in an unstrained seed layer in order to form islands or bands of material, followed by lateral epitaxial growth of a material having a different lattice parameter, so as to release the strain in the epitaxially grown layer of material. This document also mentions the possibility of transferring an initially strained seed layer onto a support via a low-viscosity layer and then of applying a heat treatment for at least partial relaxation. The lattice parameter of the seed layer thus approaches its nominal lattice parameter. Subsequent homoepitaxial growth on the seed layer makes it possible to obtain a material that is less strained than the material of the initial seed layer. The document also proposes transferring a seed layer onto a support substrate, the thermal expansion coefficient of which places the seed layer in tension at the epitaxial growth temperature so that its lattice parameter approaches that of the material grown epitaxially at this temperature.
U.S. Pat. Nos. 7,348,260 and 6,794,276 also disclose similar techniques.
However, these documents do not describe how to adapt the lattice parameter of a seed layer of an initially tensilely or compressively strained material to the point of reversing the stress and obtaining a compressively strained or tensilely strained material, respectively, depending on the desired subsequent application, especially for epitaxially growing a material having a low defect density.